1. Field of the Invention
The present invention generally relates to a demodulator structure and, more particularly, to a low complexity and low power phase shift keying demodulator structure.
2. Description of the Prior Art
Generally speaking, wireless powering systems or high receiving signal-to-noise ratio systems for the low industry, the science and medical science (ISM) band, mostly adopt the amplitude shift keying (ASK) technique. Despite the ASK demodulator structure being convenient to implement in practice, according to references 1 and 2, the demodulation way of the ASK using the envelope detection may cause the demodulator to be poor in area efficiency for the systems applied to the low ISM band.
Besides, the modulation/demodulation of the ASK may not implement the wireless transmission with a high data rate, which is only suitable for applications where data rate is of secondary importance. For those wireless systems adopting low ISM band and requiring high-data-rate transmission, such as some implantable biomedical applications, the frequency-shift keying demodulator submitted by reference 3 is another option. The technology in reference 3 gets rid of the high cost defect caused by the ASK demodulation, but the energy efficiency of the wireless transmission is compromised instead. As a result, the overall efficiency of the wireless powering system is tremendously affected.
A phase shift keying (PSK) demodulator has been submitted by reference 4. Although the PSK technique possesses higher efficiency compared to the frequency shift keying (FSK) and ASK counterparts, the receiver must have a precise reference signal in order to estimate an unknown phase and recover correct information while obtaining the phase offset. Among the PSK demodulation techniques, the Costas loop is the most realistic method. All the PSK demodulation techniques including the Costas loop require employing the closed-loop techniques, such as a phase-locked loop for carrier recovery, resulting in significant complexity.
Further, if the demodulator uses the phase-locked loop, the power consumption and implementation area of the system will considerably increase. Motivated by the limited power budget for the wireless applications mentioned above, a differential phase shift keying (DPSK) structure has been submitted by reference 5. Although the structure uses the differential coding to eliminate the demand of the closed-loop techniques such as the phase-locked loop, the structure still suffers from the problems of complexity and power consumption. Moreover, for the structure in reference 5, correct demodulation still relies on the clock signal generated by the oscillator. Owing to the phase drifting of the oscillator stemming from device imperfection, an additional compensation circuit may be needed, resulting in increase in complexity. In addition, to achieve the same bit error rate, the DPSK structure is worse in the aspect of power efficiency as compared to the binary phase shift keying (BPSK) counterpart.
Apparently, there seems to be no satisfactory breakthrough in making a better compromise between the power efficiency and implementation area. Therefore, overcoming the disadvantages of the prior arts has been of primary importance to the skilled people in the related field.
Following is the reference list:    [1] P. Mohseni and K. Najafi, “A 1-MHz, 5-Kb/s Wireless Command Receiver for Electronic Site Selection in Multichannel Neural Biopotential Recording,” Proc. IEEE 28th EMBS Conf., pp. 6241-6244, August 2006.    [2] H. Yu and R. Bashirullah, “A Low Power ASK Clock and Data Recovery Circuit for Wireless Implantable Electronics,” Proc. IEEE Custom Intergrated Circuits Conf. (CICC), pp. 249-252, September 2006.    [3] M. Ghovanloo and K. Najafi, “A wideband frequency-shift keying wireless link for inductively powered biomedical implants,” IEEE Trans. Circuits and Systems I: Regular Papers, vol. 51, pp. 2374-2383, December 2004.    [4] Y. Hu and M. Sawan, “A fully integrated low power BPSK demodulator for implantable medical devices,” IEEE Trans. Circuits and Systems I: Regular Papers, vol. 52, pp. 2552-2562, December 2005.    [5] Zhou, M., Liu, W., Wang, G., Sivaprakasam, M., Yuce, M. R., Weiland, J. D., and Humayun, M. S., “A transcutaneous data telemetry system tolerant to power telemetry interference,” Proc. IEEE 28th EMBS Conf., 2006, pp. 5884-5887.    [6] Al-Sarawi, S. F., “Low power Schmitt trigger circuit,” Electron. Lett., 2002, 38, pp. 1009-1010.    [7] Wang, G., Liu, W., Sivaprakasam, M., Zhou, M., Weiland, J. D., and Humayun, M. S., “A wireless phase shift keying transmitter with Q-Independent phase transition time,” Proc. IEEE 27th EMBS Conf., 2005, pp. 5238-5241.